Interpretive Summary: During early growth of embryonic pigs, the embryo consists of a tiny (6-10 mm) baglike sphere of cells called the trophectoderm, which is filled with fluid. The trophectoderm consists of cells that will become the placenta, while a small patch of cells on one side of the bag (called the embryonic disc) will expand to become the fetus. Late on the 11th or early on the 12th day of gestation, the trophectoderm begins a dramatic change that will cause it to become a long, stringy filament of up to a meter in length. This remarkable transformation occurs in a span of only 12-24 hours, and occurs only in certain species such as livestock animals. Since the elongation of the embryo does not occur in humans or in the model rodent systems of mice or rats, very little information about the nature of the transformation has been produced. Current theory suggests the length to which the filament grows may determine the size of the placenta, which in turn can limit the number of piglets that can fit into the sow's uterus. As this would have a direct impact on litter size, we wished to learn more about the elongation process. Gene expression in the elongating embryo was examined by comparing the RNA populations of spherical and filamentous embryos, in order to see what genes are turned on or off during the process. One important gene, cyclooxygenase-2, which is the rate-limiting enzyme in prostaglandin synthesis, was turned on during elongation. This result suggests that the appearance of prostaglandins in the uterine fluid may be a result of expression of this gene by the porcine embryo, and may play an important role in regulating placental size and therefore litter size.

Technical Abstract:
Alterations in uterine luminal fluid composition as a result of conceptus estradiol-17ß production are believed to play a significant role in the loss of 30-40% of potential pig conceptuses. Shortly after the initiation of conceptus estradiol-17ß synthesis and secretion, the conceptuses are transformed from 1 cm spheres to 2-5 cm tubular forms and finally to filamentous threads of variable length via a process known as elongation. We have attempted to characterize gene products whose expression is either initiated or terminated as the conceptus elongates. Using RNA fingerprinting, we determined that the inducible form of the rate-limiting enzyme in prostaglandin synthesis, cyclooxygenase-2, is expressed in the filamentous pig conceptus, but not in either the spherical or transitional morphologies. Furthermore, increased expression of cyclooxygenase-2 by the filamentous conceptus was associated with increases in the content of prostaglandins (particularly ostaglandin E2) found in uterine luminal fluid.